The present invention relates to a circuit for generating and/or detecting a radar signal for synthetic aperture radar (SAR) in which the radar signal is subdivided into N sub-bands which are processed in parallel in N processing channels, each of which is provided with a mixer.
In known techniques, each channel requires a specific mixer to be implemented. For example, the technique described in the work xe2x80x9cRadar signalsxe2x80x9d by Charles E. Cook, pages 161-164 (published by Martin Bernfeld, 1967) implements local oscillators all having the same center frequency for each of the channels, associated with identical compression filters, however the mixers are different for the various channels.
U.S. Pat. No. 5,870,670 (Motorola) relates to an integrated mixer in an integrated circuit for forming an image-rejection mixer. The use of such a mixer implies making an integrated circuit that is specific for each frequency band to be processed.
In addition, U.S. Pat. No. 5,862,466 (Hewlett-Packard) proposes a radio frequency (RF) mixer which reduces peaks between signals at the local oscillator frequency and at the intermediate frequency, but which can operate only in systems presenting a single and determined intermediate frequency.
Finally, U.S. Pat. No. 5,819,169 discloses a balanced dual mixer implemented on a monolithic microwave integrated circuit (MMIC) including a diode bridge and two baluns. The mixer described in that document also requires a specific integrated circuit to be implemented for each frequency band (or sub-band), and it does not make it possible to eliminate the residual unbalance of the separators and the phase mixers, nor does it eliminate mismatches between mixers which give rise to leakage at the frequency of the local oscillator (LO).
An object of the present invention is to provide a circuit that makes it possible to avoid at least one of the above-specified drawbacks, at least to a large extent.
To this end, the invention provides a circuit for generating and/or detecting a radar signal for synthetic aperture radar in which the radar signal is subdivided into N sub-bands which are processed in parallel in N processing channels, each having a first mixer for processing in-phase signals, and a second mixer for processing quadrature signals, wherein the mixers of the N processing channels are in the form of identical mixer modules each comprising at least one mixer, and wherein each module is associated with at least one external element for adjusting the mixer module in correspondence with the sub-band with which it is associated.
The mixers are advantageously symmetrical dual-switched diode mixers. Each mixer may have a dual diode mixer coupled to inlet and outlet terminals via respective inlet and outlet baluns which form part of the mixer module and which may present an external control element for adjusting amplitude balance and/or an external control element for balancing phase, the phase balancing element serving in particular to compensate for any residual leakage at the frequency of a local oscillator (LO).
External compensation capacitors can make it possible to compensate leakage at local oscillator frequencies, at least in part. Said external element is advantageously a quarterwave delay line.
The dual mixers may have a DC external control element for adjusting their RF parameters.
The dual mixers can have external devices at their outlets for adjusting amplitude and phase so as to enable image frequency rejection to be adjusted.
The mixer module can have at least one differential amplifier presenting a main differential stage having two transistors and a circuit for adjusting amplitude and phase, comprising an auxiliary differential stage having two transistors connected in parallel with the main differential stage and connected so as to inject compensation current into the drain of one of the transistors of the main differential stage, with the compensation current being adjusted by calibrating the capacitance of an external compensation capacitor interconnecting the sources of the two transistors of the auxiliary differential stage.
The mixer may be placed in a monolithic microwave integrated circuit which can also have a temperature compensation circuit and/or a summing circuit.